The demand for lidar imaging solutions has increased with the advent of autonomous vehicles and drones. Lidar uses a pulsed laser beam to probe the distance to a reflector by measuring the time it takes for the light to be reflected back to the device. This allows for very precise measurements of a given environment. However, the small wavelength of light means that rain drops, fog, smoke, sand, and other scatterers can obscure the signal. These scatterers act to reflect light back to the lidar sensor and result in false distance measurements. Some work has been done with time-gating the lidar signal to remove returns from nearby scatterers but the benefits have been limited.
For sensitive lidar devices, the optical detector can be a single-photon avalanche diode (SPAD) configured for operating in Gieger mode. These detectors are ideal for sensitive detectors because they provide a relatively large signal every time they detect a photon. However, due to the avalanche process, these detectors can only detect a single photon at a time. Every time these detectors receive a photon, the detector is ‘blind’ to other photons for a certain amount of time often referred to as ‘dead time’. In the presence of obscurants, this dead time can effectively blind the detector to photons arriving from the target. For example, backscatter from fog produces the first detected photons obscuring any targets in the scene. This problem can be addressed in part by pulsing the laser multiple times while only detecting a photon from each pulse. When this is performed enough times, eventually a histogram can be constructed of photons arriving at all times of interest. If the fog isn't too bad, returning photons from a target may eventually be detectable over the random backscatter. This serial acquisition takes a relatively large amount of time, which may fail to achieve real time identification of targets in the scene.